This invention relates to a method of and an apparatus for use in evacuating a chamber that is defined (ie, enclosed) by a glass wall that includes a port through which evacuation is effected. The invention has been developed in the context of evacuated door and/or window glazing (xe2x80x9cvacuum glazingxe2x80x9d) and the invention is herein after described in that context. However, it will be understood that the invention does have broader application, for example in evacuating cathode ray tubes, including television picture tubes, vacuum flasks, solar collector panels and panel-form display devices.
In one form of vacuum glazing, two plane spaced-apart sheets of glass are positioned in face-to-face confronting relationship and are hermetically sealed around their edges with a low melting point glass that commonly is referred to a solder glass. The space (ie, chamber) between the sheets is evacuated and the face-to-face separation of the sheets is maintained by a network of small support pillars. In typical situations the glazing may comprise glass sheets that have a surface area in the order of 0.02 to 4.00 square metres, sheet thicknesses in the order of 2.0 to 5.0 millimetres and a face-to-face spacing in the order of 0.10 to 0.20 millimeters.
The evacuation of glazing presents special problems (relative to the evacuation of many other glass objects), in that the region at which evacuation is effected should not be obtrusive in the final product and in that the evacuating procedure must be appropriate to panels having large surface areas or linear dimensions. The evacuating procedure involves heating the glazing to a high temperature in a so-called bake-out oven (whilst the chamber to be evacuated is connected to an evacuating system) in order to remove gases from surface regions of the sheets and the pillars.
In one method of implementing the evacuating procedure, a small pump-out tube is connected (ie, sealed) to the external surface of one of the glass sheets in communication with an aperture that extends through the lass sheet. The connection between the tube and the glass sheet is made using solder glass, normally at the same time that the solder glass edge-seal is made between the sheets. The solder glass sealing is normally effected at a temperature around 450xc2x0 C., that is at a temperature that is high enough to melt the solder glass and make a leak-free joint but below a temperature at which significant deformation might occur in the glass sheets.
Following completion of the solder glass sealing, the structure is normally cooled to room temperature. Thereafter, a metal evacuating head is positioned over the pump-out tube and is connected to a remote vacuum pump. The evacuating head includes a central cavity that is dimensioned to receive the pump-out tube and to which the vacuum pump is connected. Also, an O-ring containing groove surrounds the central cavity in the evacuating head and the O-ring is arranged to seal against the tube-surrounding portion of the glass sheet during the evacuating process.
The glazing structure and the evacuating head are loaded into the bake-out oven where evacuation is effected whilst the complete structure is subjected to an out-gassing temperature. Thereafter, the outer end of the pump-out tube is closed by heating the tube to its melting temperature.
A problem that is inherent in the above described evacuating procedure flows from the use of the O-ring to effect sealing between the evacuating head and the glass sheet. All elements of the structure must be heated to substantially the same temperature within the bake-out oven and, because virtually all elastomeric (ie polymeric) O-ring materials decompose at temperatures above about 200xc2x0 C., the temperature at which out-gassing may be performed is severely limited. As a consequence, in order to achieve adequate out-gassing at the lower temperatures, the procedure must be sustained for long time periods. This limits manufacturing rates and increases manufacturing costs.
A further problem of the above described procedure resides in the fact that two heating steps are required; one at a higher temperature (in the order of 450xc2x0 C.) to form the solder glass seals and the other at a lower temperature (in the order of 200xc2x0 C.) when evacuation of the chamber is effected. This two-stage process causes further increases in manufacturing time, due to the need to limit the rate at which temperature changes occur in order to avoid the induction of excessive mechanical stresses in the glazing. Furthermore, the two-stage procedure requires a greater energy consumption than would be necessary if a single-stage procedure might be adopted.
The present invention is directed to a method of and an apparatus for evacuating a chamber which avoid the need for the use of an elastomeric material to effect a seal between the evacuating head and the glass surface.
Broadly defined, the invention provides a method of evacuating a chamber that is enclosed at least in part by a glass wall that includes an evacuation port. The method comprises the steps of:
(a) covering the port and a portion of the glass wall that surrounds the port with an evacuating head having (i) a first cavity that communicates with the port and (ii) at least one further cavity that surrounds the first cavity and which is closed by the portion of the glass wall that surrounds the port,
(b) evacuating the closed further cavity or, if more than one, each of the further cavities in the evacuating head, and
(c) evacuating the chamber by way of the first cavity in the evacuating head.
The invention is further defined as providing an apparatus for use in evacuating a chamber that is enclosed at least in part by a glass wall that includes an evacuation port. The apparatus comprises:
(a) an evacuating head that is arranged to cover the port and a portion of the glass wall that surrounds the port, the evacuating head having a first cavity that is arranged in use to communicate with the port and at least one further cavity that surrounds the first cavity,
(b) a first conduit communicating with the first cavity and arranged for connecting a vacuum pump to the first cavity, and
(c) a further conduit connecting with the further cavity or, if more than one, with each of the further cavities and arranged for connecting a vacuum pump to the or each further cavity.
In the various aspects of the invention, separate vacuum pumping systems preferably are provided for connection to the first and further cavities respectively of the evacuating head. However, evacuation of the or each further cavity may be effected at a significantly lower level (ie, to produce a less complete vacuum) than that which is applied to the first cavity.
As indicated previously, the invention has application to the fabrication of evacuated glazing and, in this application, the chamber to be evacuated will comprise a space that is defined by two edge-sealed glass sheets. One of the sheets will be provided with a pump-out tube to form the port through which evacuation is effected, and the first cavity of the evacuating head will be shaped and dimensioned to receive the pump-out tube.
With this preferred aspect of the invention in mind, the invention may be further defined in terms of a method of forming evacuated glazing wherein the evacuating process is effected whilst simultaneously effecting heat induced out-gassing of the chamber. The out-gassing procedure most preferably is commenced at a temperature that is slightly less than (typically 50xc2x0 C. to 100xc2x0 C. less than) that at which edge sealing of the glass sheets is completed.
The invention will be more fully understood from the following description of preferred embodiments of an apparatus for and a method of implementing the invention. The description is provided with reference to the accompanying drawings.